Electric storage devices such as lithium-ion batteries, nickel-hydride batteries, and other secondary batteries (storage batteries) are growing in importance as power sources for use in vehicles or power sources for personal computers and portable terminals. In particular, lithium ion batteries are expected to be used as high-output power sources for use in vehicles. Lithium-ion batteries are light in weight but high in energy density. A typical example of a structure of a secondary battery is a sealed-structure battery (sealed battery) in which an electrode assembly and an electrolyte are accommodated in a casing and the casing is sealed.
If such a battery is charged by a charging device with a defect in the battery or with a malfunction in the charging device, an electric current at a level that is higher than a normal level may flow through the battery, as a result of which the battery may be brought into an overcharged condition. If there is an abnormality such as an overcharged condition in the battery, gas may be generated within the sealed casing. A pressure inside the casing may rise to such an extent that a pressure of the gas may cause the battery (casing) to inflate or cause the casing to be damaged. As measures against such defects, there has been proposed a battery including a current interruption device. The current interruption device ensures the safety of the battery by interrupting the electric current when an internal pressure of the sealed casing abnormally rises. Such technologies are disclosed, for example, in Japanese Patent Application Publication No. H6-215746, Japanese Patent Application Publication No. 7-254401, Japanese Patent Application Publication No. H9-129214, and Japanese Patent Application Publication No. H10-233199.
A conventional current interruption device is described with reference to FIGS. 11A and 11B. A current interruption device 50 is provided near a positive electrode of a sealed secondary battery. As shown in FIG. 11A, the current interruption device 50 includes a sealing cover body 52, an inverting plate 53, and a lead attachment plate 54. The sealing cover body 52 has a conduction property, and is connected to an external terminal 51. The inverting plate 53 has a conduction property, and is disposed on an inner side of the sealing cover body 52. The lead attachment plate 54 is has a conduction property, and is disposed on an inner side of the inverting plate 53. The lead attachment plate 54 is made of metal, and adheres to a center portion (contact portion) 56 of the inverting plate 53. The lead attachment plate 54 has an air hole 55 formed therethrough so that an internal pressure of the battery acts on the inverting plate 53. An arrow 57 indicates a current path through which an electric current flows when the battery is in a normal condition. When the battery is in a normal condition, an electric current flows serially to the external terminal 51 through the lead attachment plate 54, the inverting plate 53, and the sealing cover body 52. It should be noted that an outer circumference portion of the lead attachment plate 54 is supported by a supporting member 58. The supporting member 58 is made of an insulating material.
If the battery is brought into an abnormal condition such as an overcharged condition, gas is generated within the sealed casing, with the result that an internal pressure of the casing rises. If the battery is brought into an abnormal condition, the internal pressure of the easing acts on the inverting plate 53 through the air hole 55 of the lead attachment plate 54 as shown in FIG. 11B. An adhering portion at the center portion (contact portion) 56 of the inverting plate 53 breaks, and the inverting plate 53 gets inverted upward. The current path 57 gets broken, and the electric current gets interrupted.